text: For most substances, the solid form is denser than the liquid form.  Water is special in that the solid (ice) is less dense than liquid water due to the irregular shape of the crystal, due to the bent shape of the molecule.  THis makes it so that the ice floats on top of the lake, rather than sinking to the bottom.  In this way, the ice actually serve to insulate the lake and prevent the lake from freezing entirely, making the flora and fauna beneath able to survive the winter.